Lesson guide · Lesson 18 · Grades 4–8 · 7 min

The nitrogen cycle, step by step

How two kinds of bacteria pass fish waste along, step by step, until it becomes plant food.

Open the lessonPrintable worksheet

Students will

  • Name the steps from fish waste to plant food: ammonia, nitrite, nitrate.
  • Explain which job each kind of bacteria does.
  • Explain why nitrate is safer for fish than ammonia or nitrite.

New words

ammonia
A waste fish give off into the water. Too much of it hurts fish.
nitrite
What one kind of bacteria makes from ammonia. It can hurt fish too.
nitrate
What a second kind of bacteria makes from nitrite. Plants use it as food.
nitrogen-cycle
The loop that turns fish waste into plant food, step by step.

Talk about it

A relay race needs every runner. What happens to the cycle if one kind of bacteria is missing?

Check questions and answers

  1. Which one is safest for fish?

    • Nitrate (answer) Right. Nitrate is much safer for fish, and plants use it as food.
    • Ammonia Not quite. Ammonia is the waste that hurts fish when it builds up.
    • Nitrite Not quite. Nitrite can hurt fish too. It is only halfway there.

  2. What do the Munchers make?

    • Nitrite (answer) Right. Munchers turn ammonia into nitrite.
    • Nitrate Not quite. Nitrate comes next. The Finishers make it.
    • Fish food Not quite. People feed the fish. Munchers work on the waste.

  3. Why does a system need lots of Finishers?

    • They turn nitrite into safer nitrate (answer) Right. They finish the job, so nitrite doesn't build up.
    • They feed the fish Not quite. People feed the fish. Finishers work on nitrite.
    • They keep the water warm Not quite. Finishers can't change the warmth. They change nitrite.

Sources

  • Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. Small-scale aquaponic food production: Integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper No. 589. Food and Agriculture Organization of the United Nations, Rome, 2014.
  • Rakocy, J. E., Masser, M. P. and Losordo, T. M. Recirculating Aquaculture Tank Production Systems: Aquaponics, Integrating Fish and Plant Culture. SRAC Publication No. 454. Southern Regional Aquaculture Center, November 2006 revision.
  • Hargreaves, J. A. and Tucker, C. S. Managing Ammonia in Fish Ponds. SRAC Publication No. 4603. Southern Regional Aquaculture Center, December 2004.
  • Durborow, R. M., Crosby, D. M. and Brunson, M. W. Nitrite in Fish Ponds. SRAC Publication No. 462. Southern Regional Aquaculture Center, June 1997 (revised).
  • McGee, M. and Cichra, C. Principles of Water Recirculation and Filtration in Aquaculture. FA-12. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published October 1988, reviewed April 2000.
  • Deer, C., Hu, B., Dunn, B. and Dusci, J. Nitrification and Maintenance in Media Bed Aquaponics. Fact sheet HLA-6729. Oklahoma Cooperative Extension Service, Oklahoma State University, October 2021.
  • Daims, H., Lebedeva, E. V., Pjevac, P., Han, P. et al. Complete nitrification by Nitrospira bacteria. Nature 528, 504-509 (2015).
  • van Kessel, M. A. H. J., Speth, D. R., Albertsen, M., Nielsen, P. H., Op den Camp, H. J. M., Kartal, B., Jetten, M. S. M. and Lücker, S. Complete nitrification by a single microorganism. Nature 528, 555-559 (2015).